1. Technical Field
This invention relates to systems and methods for determining the position of a mobile unit using ultrasound.
2. Background Information
There are many situations in which it is desirable to be able to determine the location of a moveable object, such as a person or an item of equipment, within an area, such as a hospital ward.
It is known to provide a network of static base stations, e.g. attached to ceilings or walls, and to attach tags or similar devices to moveable objects, where the tags communicate with one or more of the base stations through ultrasound signals so as to allow the location of the tagged object to be determined. For example, such a system may be used to determine what room the object is in. Ultrasound is well suited to this purpose as it is undetectable by humans. It also travels much more slowly through air than radio waves do, and attenuates more rapidly, especially in the presence of structural building members such as walls, ceilings and floors. This can facilitate proximity detection using time of flight information or received signal strength.
U.S. Pat. No. 7,864,633 describes a system for determining the position of a mobile acoustic transmitter unit. The transmitter unit, which can be attached to an object to be monitored, transmits a frequency-shift key (FSK) encoded acoustic signal. A detector unit may be installed in a room of a building. The transmitter unit's position can be determined by comparing the received signal strength of the signal at two such detector units to determine which of the detector units is closer to the transmitter unit. In noisy environments, chirp FSK may be used instead of conventional FSK.
Such an approach has certain limitations, however. In particular, as the number of mobile transmitter units in the system increases, the average rate of position updates for each transmitter unit (i.e. the number of position estimates per minute) will typically decrease because each unit has to wait longer for silence before it can start to transmit. Moreover, interference between units is still possible, for instance if two units start transmitting substantially simultaneously. To cope with noise, digital signal processing (DSP) circuits are used for processing received chirp FSK signals; such circuits are typically expensive and power-hungry.
The present invention seeks to provide an alternative approach.